Until now the most precise beam energy measurements have been calibration measurements of the dipole fields in the bending magnets with Hall probes. These measurements have led to a presumed energy precision  of .
In order to get the beam energy with a precision better than new measures have to be taken. One possibility is to measure the resonant excitation frequency at which the spins ensemble gets resonantly depolarized . This technique has recently been used successfully at ELSA , BESSY II  and ALS . The next sections will discuss polarization of electron beams, depolarizing effects and will explain how the polarization level of an electron storage ring can be determined.